Oxygen is a commodity chemical in the industrial gas industry. It has numerous applications, including wastewater treatment, glass melting furnace, and steel industry. One of the most common methods of oxygen production is by cryogenic distillation of air. However, this technology is not competitive for small size oxygen plants (&lt;100 TPD O.sub.2). The technology of choice for this size range is adsorptive separation. There is a need in the marketplace to produce oxygen at low capital and energy costs by adsorptive gas separation.
Adsorptive processes are extensively used in the industry to produce oxygen from air for small size oxygen plants (&lt;100 TPD O.sub.2). There are two major categories of these processes; pressure swing adsorption processes and vacuum swing adsorption processes. The pressure swing adsorption (PSA) processes carry out the adsorption (feed) step at pressures much higher than ambient pressure and adsorbent regeneration at pressures close to ambient pressure. The adsorbent beds go through secondary process steps such as pressure equalizations, repressurizations, blowdowns, and purge or various combinations of these during the cycle.
PSA processes tend to be energy intensive and more suitable for smaller oxygen plants producing less than 40 tons of oxygen per day and preferably less than 20 tons of oxygen per day. Primary reasons for high energy consumption in PSA processes are: (1) O.sub.2 recovery from these processes is low, and (2) the entire feed stream has to be compressed up to the adsorption pressure. These inefficiencies are somewhat circumvented in vacuum swing adsorption (VSA) processes. In these processes, adsorption is carried out at pressure close to ambient pressure, and adsorbent regeneration is carried out at sub-atmospheric pressure levels. The adsorbent beds go through several secondary steps, such as; pressure equalizations, repressurizations, and purge, or various combinations of these during the course of the cycle with the primary aim of increasing oxygen recovery and reducing adsorbent inventory per unit of product gas.
In both PSA and VSA processes a typical adsorber configuration consists of an adsorption bed having an inlet for admitting air and an outlet for removing oxygen rich product. Per U.S. Pat. No. 4,892,565, the bed normally contains two adsorbent layers. The first layer comprising approximately 15% of the adsorber volume and being located adjacent to the inlet end of the adsorber is specific to the selective removal of water and carbon dioxide. The second layer comprising approximately 85% of the adsorber volume and being located adjacent to the outlet end of the adsorber is specific to the selective removal of nitrogen. As a reflection of its functionality, the first layer is commonly called the pretreatment layer. The second layer is similarly called the main adsorbent layer, comprising, in the present invention, air separation adsorbent which separates nitrogen from oxygen in the feed air. Typical pretreatment adsorbents include alumina, NaX zeolite and silica gel. Typical main bed adsorbents include NaX, CaX, CaA, MgA, and LiX zeolites. U.S. Pat. No. 5,114,440 teaches that the typical pretreatment layer comprises approximately 15% of the adsorber volume while the main adsorbent layer comprises approximately 85%. The pretreatment material is usually selected on the basis of its working capacity for water and CO.sub.2 under the specific VSA/PSA cycle conditions. The main adsorbent material is usually selected on the basis of its cost and its working selectivity and capacity for nitrogen over oxygen under the specific VSA/PSA cycle conditions. The VSA/PSA cycle conditions which affect this decision normally include feed gas temperature, adsorption pressure and desorption/evacuation pressure. Lower capacity adsorbents, such as NaX, are generally favored at low feed temperature and high adsorption pressure conditions, while CaX, CaA, MgA, LiX are generally favored at high feed temperature and low adsorption pressure conditions. Selection of the ideal adsorbent is normally based upon tests done in laboratory size adsorbers, 2" to 4" in diameter. Frequently however, there is a significant performance loss (lower O.sub.2 recovery and production) in the full-scale plant, relative to the lab scale. The primary cause of the performance loss is an axial temperature profile that develops in the main adsorbent layer during full scale plant operation. It is not uncommon for there to be a 100.degree. F. difference between the lower (inlet) cold end and the upper (outlet) hot end of the main adsorbent layer.
This temperature profile is detrimental because commercially available zeolites do not function optimally over such a wide range of temperatures. High capacity zeolites such as Ca X-zeolite lose selectivity and develop a high capacity for O.sub.2 at low temperatures (below 50.degree. F.). This O.sub.2 is subsequently rejected to waste on the evacuation step, thereby lowering product O.sub.2 recovery. Such zeolites are also difficult to regenerate with vacuum when they are cold. Specific vacuum power increases as a result. Lower capacity zeolites, such as NaX, do not adsorb enough O.sub.2 in the cold part of the bed to suffer from low recovery. However, they consequently suffer from low capacity and selectivity in the hot section of the bed (above 50.degree. F.).
These losses in performance were originally documented by Collins in U.S. Pat. No. 4,026,680. Several efforts have been made to overcome them by varying the capacity of adsorbents in a bed per European Application No. 0 512 781. Differing capacities of Ca A-zeolite are suggested in U.S. Pat. No. 5,114,440. An obvious solution is to raise the feed temperature. This does raise the minimum temperature by effectively shifting the entire profile upwards. The resulting high temperatures at the product end of the bed causes a loss of adsorbent capacity, thereby partially or completely counteracting the benefit of the increased minimum temperature. Another proposed solution is to employ conducting rods or plates, axially aligned in the bed to moderate the temperature profile by heat conduction per U.S. Pat. No. 4,026,680. This solution is difficult and expensive to install in full-sized adsorber beds. The operating pressure envelope can be reduced, but at a cost to sieve productivity. Japanese Patent Appln. No. 4-293513 discloses a process using adsorption beds layered with only Ca A-zeolite followed by Ca X-zeolite to improve process performance based upon better management of heats of adsorption.
Additional patents of interest include; U.S. Pat. No. 3,636,679; U.S. Pat. No. 4,756,723; U.S. Pat. No. 4,892,565 and U.S. Pat. No. 5,152,813.
Despite the prior art, a need still exists for process technology which will eliminate the detrimental effects of full-scale adiabatic temperature profiles in O.sub.2 PSA/VSA processes at minimal cost and such that the performance loss between laboratory size adsorbers and full scale adsorbers is eliminated. This is the object of the present invention.